Method of modulating highfrequency transmitters



1931 -H. WEHRLIN El AL 2,090,82G

METHOD OF MODULATING HIGH FREQUENCY TRANSMITTERS Filed June 22, 1953 5Shets-Sheet 1 Film 1931 H. WEHRLIN ET AL 2,090,82G

METHOD OF MODULATING HIGH FREQUENCY TRANSMITTERS Filed June 22, 1933 3Sheets-Sheet 2 Au 24, 1937. H. WEHRLIN ET AL 2,090,820

METHOD OF MODULATING' HIGH FREQUENCY TRANSMITTERS Filed June 22, 1933 3Sheets-Sheet 5 arm if ney Patented Au 24, 1937 PATENT OFFICE METHOD OFMODULAT ING HIGH- FREQUENCY TRANSMITTERS Hans Wehrlin,

Berlin-Lichterfelde, and

Leo

Fungs, Brunswick, Germany; said Wehrlin as- Slgnor to C. LorenzAktiengesellschaft, Berlin- Tempelhof, Germany 7 Application June 22,1933, Serial No. 676,999 /2 In Germany June 22, 1932 1 Claim.(01.179-171) The invention relates to a method of modulating highfrequency transmitters and more particularly to a method with which theamplitude of the carrier frequency varies in dependence on the intensityof modulation.

In accordance with one object of the invention, the carrier frequency ofa certain minimum amplitude, preferably corresponding to the lower endof the straight portion of the grid voltageplate current characteristicof the modulating tube, is transmitted during non-modulation periods andwhen the modulation intensity is quite low, namely below a certainminimum value.

In accordance with another object of the invention, the carrierfrequency transmitted does not exceed a predetermined maximum amplitude,preferably corresponding to the upper end of the said straight portionof said characteristic; so that even though the modulation intensitiesmay exceed a certain value the amplitude of the transmitted carrierfrequency is unaffected thereby.

According to another object of the invention, in a high frequencycarrier wave signalling system employing a transmitter and areceiver thenon-linear demodulation characteristic of the receiver is neutralized orcompensated at the transi mitter. The arrangement is preferably suchthat the amplitudeof the carrier frequency is not precisely accommodatedto the: demodulation curve but that the displacing of the working pointis limited to a certain valuein downward direction and that the workingpoint shall remainat this value even if the intensity of modulation isvery small or equal to zero. i o If the working point is displaced thenthe main saving in energy is attained by the transmitter not operatingcontinuously with the: maximum carrier current. From acertain-lowervalue a further downward displacement of the working point does notbring about a further essential saving in energy. In general this valuecan be as.- sumed to be approximately at or of its maximum value. Thisdownward displacing should be effected to such adegree only that thereceivers shall not yet be subjectedto distortions.

Moreover the arrangements required for dis placing the working point aresimpler if, as stated before, in accordance with the invention the da-50 modulation curve of the receiver need not be observed strictly in thelower range that requires the larger expenditure in means, butshall beso only I inthe upper range which is more rectilinear. In addition, theexisting receivers are more or less '55 in conformity with each other'as regardsjthe upper part of the demodulation curve, whilst in mostcases they are so to a less degree with respect to the lower partthereof.

Furthermore, the carrier current which is weak during the pauses may beused with advantage for transmitting a pause signal. If, however, nodirect pause signal is sent, then nevertheless there is the advantagethat the receiver is more easy to adjust to the transmitter than if thecarrier frequency of the transmitter were missing entirely.

The invention will be understood from the following'description and beparticularly pointed out in the appended claim, reference being had tothe accompanying drawings in which Fig. 1 is a diagram illustrating thefundamental idea on which the novel method is based. Fig. 2 is a wiringdiagram of an arrangement adapted for carrying out the novel method.Fig. 3 shows a circuit arrangement of an electron tube that is useful'inconnection with the invention. Fig. 4 represents a slight modificationof the arrange.- ment disclosed in Fig. 3. Fig. 5 illustrates thecharacteristic curve of tubes which are arranged as shown in Figs. 3 and4. Fig. 6 is a diagram relating to a certain mode of modulation. Fig. 7is a diagram relating to a known mode of transmitter operation. Fig. 8is a diagram that relates to the mode'of working of the noveltransmitter. Figs. 9 to 11 each represent a circuit arrangement adaptedforv carrying out the invention.

In'Fig. 1' there is'shown as a function of the time t the evolution thecarrier current undergoes on a transmission, such as a musical one,taking place. This representation in its left-hand portion also showsthe mean demodulation curve. This curve represents the rectified currentlg as a function of the high-frequency voltage U. The scales for i and Ubeing the same, this-curve enables directly to read oif the rectifiedcurrent that corresponds to a definite antenna. current. Hence also theworking point for any carrier current can be ascertained. During thepauses of modulation; specially indicated in the'figure, the carriercurrent in accordance with the invention does not approach the zerovalue but the value 10, This value corresponds to a working point thatas far as possible belongs to the part where the slope of the modulationcurve is approximately constant' or at least essentially different fromzero. From this value the carrier current increases in dependence on'themodulation intensity, in order then to decrease, to the value ie duringthe pauses of modulation and with small values of modulation. In theexample represented the minimum value is is about of the maximum valueimax.

The arrangement shown in Fig. 2 is for the case of modulation of gridpotential with a separately excited transmitter. M denotes themicrophone, V1 the modulation amplifier from which the normal modulationvoltage acts on the transmitter over the transformer T1. Over a secondamplifier V2 and the rectifier G the continuous voltage for displacingthe working point arrives. The distortion for the accommodation to thedemodulation curve of the receiver may here be efiected either in theamplifier V1 or in amplifier V2. In one case only the modulation curveis distorted, whilst in the other case the displacing of the workingpoint too is influenced by the distortion. For producing thiscompensating distortion at the transmitter an electron tube of thevariable mu type is preferably employed. The special construction of thegrid of such a tube causes a gradual change in the mutual conductance'ortransconductance of the tube near the lower portion of the platecurrent-grid voltage characteristic curve. Different degrees ofamplification are then obtained in accordance with the different gridbiases. These tubes above all have the advantage that they arecontrolled almost without power input, since the working points lie inthe range of negative grid bias.

Figs. 3 and 4 relate to the working with tubes of this kind, Fig. 3shows a capacity-resistance coupling, Fig. 4 a transformer coupling.Should the tube also contain a screen or protective grid, it is'advantageous to connect said grid to the anode since, due to the lowfrequencies at which the'tube is operating, a more favorablerelationship is thereby obtained'between the internal resistance of thetube and the impedance of the output circuit.

Since, as shown in Fig. 5, the characteristic of 'such tubes has amaximum due to the overmodulation effected with small negative gridpotential, it is advantageous to provide for a constant'bias AEg inaddition to the variable control voltage'fin order always to work onthat part of the characteristic curve which is situated on the left-handside with respect to this bias.

The undistorted modulation currents of the microphone M (Fig. 2) areconducted to the input Eg of the arrangement represented in Fig. 3 orFig. 4, while from the output EA the correspondingly distortedmodulation voltages are derived.

. In the arrangement shown in Fig. 2 the mini mum value of thecarrier'current is in accordance with. the invention regulated by meansof the potentiometer P. The potentiometer P is the means by which thegrid of the tube RS is biased. When the swing of the grid excitationvoltage applied through the transformer T; from the source of continuousoscillations O is greater than that required to reduce the plate currentto a value below the straight portion of the plate current-grid voltagecharacteristic, the grid biasing potential is the controlling factor inthe alternating or carrier current output of the tube. The biasingpotential is regulated so that with zero or low modulating voltagesthere is at least a minimum value is of antenna carrier current. On anyincrease in intensity of the modulating voltage above the amountrequired to produce theminimum carrier current, there is a decrease inthe biasof the tube RS thereby permitting an increase in the carriercurrent transmitted. To i1lustrateif there is no modulating voltageapplied to the full wave rectifier G the biasing voltage applied to tubeR is only that due to the battery C and this bias is of such value thatthe plate current of tube R, flowing through the resistance ofpotentiometer P, produces the necessary voltage drop properly to biasthe tube RS. If a modulating voltage is next applied to the rectifier G,the rectified current flowing through the resistance B produces avoltage drop which, when added to the voltage of the battery C, causesthe grid of tube R to become more negative. This,

in turn, reduces the plate current of tube R and thereby causes adecrease in the negative voltage across potentiometer P. This decreaseacross potentiometer P causes an increase in the antenna carriercurrent, as previously explained.

According to the invention, the biasing voltage of tube RS may be soshown that the minimum value of antenna current i0 falls below thestraight line portion of the characteristic curve. Assuming such a case,the circuit may be so adjusted that on modulation taking place theresulting change in bias on tube RS is in such a direction and of suchvalue as to increase the minimum carrier current to the value is (Fig.6) which is the value at which the plate current-grid voltagecharacteristic begins to be rectilinear. Any further increase inmodulation intensity causes the carrier current to vary along therectilinear portion of the characteristic curve, in accordance with themodulation intensity.

In some cases it is advantageous to limit the displacing of the workingpoint not only downward but also in upward direction. By so doing theundesirable effects of overmodulation can be obviated. For in the eventof such modulation peaks the working point rises beyond the normal ihalf-value and hereby the unpleasant incidents due to overmodulation arestill increased.

In Figs. 7 and 8 there are represented in a quite diagrammatic mannerthe conditions of a transmitter without limitation of the Working point(Fig. '7) and with a limitation of this nature (Fig. 8) as set forth bythe invention. The transmitter operating in accordance with Fig. '7proves to be much overmodulated, whilst the transmitter constructedaccording to Fig. 8, i. e. according to the invention shows a uniformmodulation curve. The limitation of the working point can here beeffected by all devices that operate with any bent or saturated voltagecharacteristic.

With the arrangement represented in Fig. 9, in the input circuit of anamplifier V a microphone M is included the amplified currents of whichare on the one hand conducted to a rectifier G and on the other hand ledto an intermediate repeater ZV. The rectifier G as well as theintermediate repeater ZV act upon a main transmitter HS influenced by acontrol transmitter St. Disposed in the output circuit of the rectifierG is a resistance R to which a glow discharge lamp L is connected inparallel. A potentiometer P, connected in advance of the lamp L, and theauxiliary potential supply E permit of adjusting the ignition voltage ofthe lamp L to a desired value. Whenever the modulation exceeds a certainvalue and a certain voltage at resistance R is exceeded thereby, thelamp L is ignited and thereby acts to limit the voltage so that thiscannot rise beyond a certain value. The adjusting is here made so thatthe limitation occurs on the half-value becoming attained. The measuringinstrument J included in the circuit of lamp L may serve for supervisingthe overmodulation. The lamp L may serve the same purpose. When it islit this indicates that the transmitter is fully modulated. If theignition voltages used are low, a thermionic gasdischarge tube may beemployed instead of the glow discharge tube.

The circuit arrangement shown in Fig. 10 differs from that beforedescribed in this that a tube RR. having a curved characteristic isemployed. In the grid circuit of this tube a resistance R is disposed sothat on the modulation increasing an increasing grid bias is conductedto the tube. The bend of the tube characteristic acts to limit the workof the tension E derived from the anode side, this tension in its turninfluencing the grid bias of the transmitter.

The arrangement illustrated in Fig. 11 operates with a relay Re, thatresponds whenever a certain voltage is exceeded. On the relay respondingthe contact 11 is opened and a constant grid bias is supplied overcontact Z2 as long as the modulation amplitudes which correspond to thehalf-value continue to be exceeded.

The arrangements represented in Figs. 9 to 11 may be employedimmediately with the arrangement shown in Fig. 2, for influencing thegrid bias of the transmitting tube RS.

What is claimed is:

In a carrier Wave signalling system employing a transmitter fortransmitting a modulated wave derived from a signal and a carrier, and areceiver having non-linear demodulation characteristics for demodulatingsaid wave to reproduce said signal, the method of deriving from saidsignal and said carrier the modulated wave to be transmitted whichcomprises distorting the signal amplitude to compensate for thenon-linear demodulation characteristics of the receiver, varying themean carrier intensity in accordance with the mean signal intensity,limiting said Varying of said carrier intensity in such manner that thismean intensity does not fall below a fixed positive value duringintervals when the mean signal intensity is zero, further limiting saidvarying of said carrier intensity so that said intensity remains at afixed upper value during intervals when the mean signal intensity isabove a predetermined maximum value, and modulating the carrier socontrolled in conformity with the instantaneous amplitudes of thedistorted signal so that the instantaneous intensities of said carrierare determined by the instantaneous amplitudes of said distorted signalwhile the mean intensity of said carrier depends upon the meancontrolled intensity of said carrier before modulation.

HANS WEHRLIN.

LEO PUNGS.

